In North America, trees for pomaceous fruits, such as apples, pears, quince, and other members of the Rosaceae family, are widely afflicted with the disease known as fire blight. Although indigenous to North America, this disease has more recently gained a foothold in Europe and now is of considerable concern on both sides of the Atlantic Ocean.
Fire blight is a bacterial disease caused by the infection of pomaceous fruit trees with the bacterium Erwinia amylovora. This bacterium can be disseminated from one tree to another by rain, wind, insects, birds, and man. Generally, infection occurs through natural openings in the tree, particularly blossoms. This causes blossoms first to appear water soaked, then to wilt and shrivel, and finally to turn black or brown. The disease then spreads to other parts of the tree, including branches, the trunk, and roots. This disease is manifested in tree limbs, trunks, and roots as cankers from which liquid oozes to spread the disease. Fire blight on twigs and suckers of fruit trees causes shoots,
Fire blight not only destroys the current year's crops but can also have a long-term impact. Blossom infection will reduce the current season's crop by killing fruit. In addition, twig blight destroys wood that could bear fruit spurs the following season. In pears and quinces, as well as many apple cultivars and rootstocks, blight can destroy large limbs or even an entire tree. In view of fire blight's potentially devastating effect on pomaceous fruit crops, the need exists to combat that disease.
It has been found that pear cultivars and many apple cultivars are particularly susceptible to fire blight. Nevertheless, both types of cultivars have some forms which are more resistant to fire blight. Not only do the fruiting scions have varying susceptibility, but so do the rootstocks for apple. One approach to combating fire blight is to breed cultivars and rootstocks for pomaceous fruit trees which are resistant to fire blight. Such programs, however, require trial and error and long periods of time to yield trees with fire blight resistance. In addition, a very limited number of apple and pear cultivars are responsible for a large portion of annual production. These cultivars are prized by consumers, supermarkets, and growers for their appearance, quality, flavor, storability, and production characteristics. To retain varietal characteristics and to introduce disease resistant genes by sexual breeding is virtually impossible, because the long generation time and self-incompatibility of apples and pears make backcross programs astronomically longterm and expensive.
Another approach to combating fire blight is by following horticultural practices which minimize the disease's outbreak. It has been found that reducing soil moisture and maintaining a balance of fertilizer nutrients can control fire blight infection and propagation. Although such approaches can be helpful, they are not capable of eliminating outbreak of the disease.
It is also possible to treat fire blight by removing cankers and blighted branches from infected trees, preferably during the winter when the disease is dormant. Equipment for carrying out such procedures must, however, be carefully sterilized to prevent the disease from being spread. Moreover, this approach cannot completely eradicate the disease, because areas with small cankers or internal infection may escape detection.
Trees infected with fire blight can also be periodically sprayed with copper compounds or antibiotics to control fire blight. The application of copper compounds has not achieved wide acceptance, however, because it is often ineffective and causes fruit russeting. The use of antibiotics, particularly streptomycin, is more effective and less injurious to fruit than copper compounds. However, Erwinia amylovora has developed resistance to streptomycin in many states where it has been used, including California, Oregon, Washington, Missouri, and Michigan. Further, an antibiotic program is expensive and many countries in Europe prohibit its use.
Biological control of fire blight has also been attempted. Such efforts have been particularly directed to developing organisms antagonistic to Erwinia amylovora. Biological control studies indicate that such techniques have potential usefulness in controlling fire blight, but none of the tested procedures are sufficiently effective or developed to replace chemical treatments.
In view of the deficiencies of present techniques of combating fire blight in pomaceous fruit, the need remains for an effective treatment procedure.